Engineers: CLT has the potential to revolutionise mid-rise buildings

The engineers who specialise in building with cross-laminated timber (CLT) do not see much of its future in prototype towers. Instead, they say, they see it as a fundamental part of the mid-rise building market.

“Some of those pioneering early CLT buildings are really almost like a partial R&D project in disguise,” John van de Lindt from Colorado State University (CSU), Colorado, United States (U.S.), said, noting that he and some colleagues have foreseen that the market’s maturation would depend on the integration of research-driven CLT standards into present building codes.

“If you’re going to just do a two-storey residential home, you have a perfect design code pathway to do it,” Shiling Pei from the Colorado School of Mines, said to The Architects Newspaper. “But if you want to go taller, especially if you want to go above 85 feet (25.9 metres) – that is currently International Building Code (IBC) for Type IV, heavy timber – then you have to do something else such as conducting a number of tests to try to convince the local building-code officials.”

Pei credits the 2011 CLT Handbook by FPInnovations, a non-profit organisation from Canada, with beginning the timber revolution with the introduction of the PRG 320, a performance-based standard developed with input from the APA: The Engineered Wood Association and the American National Standards Institute. Updated twice, once in 2012, and again in 2017, the standard offers detailed specifications of the composition, strength, and stiffness of the CLT products, among other properties.

As CLT was first developed in Europe, where seismic activity is milder, transferring the CLT technology to Canada and the U.S., especially for larger projects, needs standards that address lateral forces. Moreover, CLT’s seismic design provisions in the American Society of Civil Engineers’ (ASCE) code book, ASCE 7: Minimum Design Loads for Buildings and Other Structures should also be updated, especially its response-modification coefficient, or R factor, according to The Architects Newspaper.

“To make CLT economically competitive, it really needs to have these seismic performance coefficients in the code, so that people don’t have to get special permission every time they want to use it,” van de Lindt continued, who also noted that engineers incurred reviewing costs every time they sought permission to build with CLT. “With CLT, everything rotates like a rigid body under seismic stress. Panels do not deform enough to dissipate energy and suck load right into them…For a steel special moment frame that’s detailed for seismic, it can be an R of eight, which has a lot of ductility.”

But according to van de Lindt, adding concrete or steel lateral systems to form hybrids, such as Brock Commons in the University of British Columbia, British Columbia (B.C.), Canada, wastes the construction speed CLT is known for, and is calling for performance-based modelling rather than prescriptive tables for very tall wood buildings that stand at 20 stories or higher as they “will always require review, at least in our lifetime.”

“You get the best out of both materials,” Andre Barbosa from the School of Civil and Construction Engineering and Tallwood Design Institute in Oregon State University, Oregon, U.S., agreed. “You have the CLT that’s lighter; its strength-to-weight ratio is very, very good. You get the concrete that allows you to go to longer spans, but also it creates that natural barrier for smoke and essentially for fire across floors.”

He also stated that the present methods of addressing the timber’s susceptibility to moisture and insects are adequate, adding that further study is needed for long-term, deflection (creep) in tall CLT buildings.

Pei and his colleagues, supported by the National Hazards Engineering Research Infrastructure Tall Wood programme under the National Science Foundation (NSF), have recently completed the construction of a two-storey prototype to test on the world’s largest shake table at the University of California San Diego (UC San Diego). The table simulated 14 quakes of varying severity, from a mild quake to a “maximum credible earthquake,” an event that occurs once every 2,500 years.

According to Pei, “the building essentially received no damage, and we don’t need to repair anything.” He also noted that the CLT walls outstripped their concrete and steel equivalents in terms of resilience.

“Flammability is a concern very often expressed, but an easy one to dismiss,” Lech Muszynski, an associate professor of wood science and engineering at Oregon State University’s College of Forestry, said. Studies have shown that the contrary idea that charring the wood actually produces an insulating layer that slows the burning, making the flames advance in a predictable patterns and saving enough wood to pass fire-resistance tests that last up to two hours.

“I’ve done some testing on unprotected CLT assemblies here in the states, large-scale floor and wall assemblies; there is a large library of similar test being done in Europe in the past,” Muszynski added.

Moreover, in practice, CLT is usually encased in gypsum board, which adds another hour or more to its fire-resistance rating.